1. Field of the Invention
This invention relates to a process for making perchloroethylene and hydrogen chloride by noncatalytic thermal chlorination of hydrocarbons or their partially chlorinated derivatives in the presence of hydrogen and carbon tetrachloride. In particular, it relates to a process for production of perchloroethylene and hydrogen chloride by the noncatalytic thermal chlorination of C.sub.1 to C.sub.3 hydrocarbons and/or their partially chlorinated derivatives in the presence of hydrogen using carbon tetrachloride as a reactive diluent under conditions which maximize consumption of carbon tetrachloride supplied to the process from an external source, while minimizing the production of heavy ends, such as hexachlorobenzene and other tarry products.
2. Background of the Invention
When conventionally manufacturing perchloroethylene by chlorination of hydrocarbons and/or their partially chlorinated derivatives, substantial amounts of carbon tetrachloride are also obtained. In addition, substantial quantities of undesirable hexachlorinated products such as hexachloroethane, hexachlorobutadiene and hexachlorobenzene, hereinafter referred to as heavy ends, are formed. Carbon tetrachloride, however, is thought to be among the halocarbons which cause destruction of the ozone layer and is therefore coming to be considered a relatively undesirable byproduct which is finding fewer and fewer commercial uses. More particularly, demand for it as a feedstock in producing fully halogenated chlorofluorocarbons, which previously provided an important market for carbon tetrachloride, has greatly decreased because of the environmentally deleterious nature of such chlorofluorocarbons. Because of the undesirable environmental attributes of carbon tetrachloride and products made therefrom, regulations governing the production and use of carbon tetrachloride are expected to result in a major decrease in the demand for carbon tetrachloride over the next decade. On the other hand, the more ecologically benign chlorinated hydrocarbons, notably perchloroethylene, are expected to remain in high demand because of their many practical uses, both as a solvent and as a starting material for the production of other chemicals. The present invention addresses this situation by providing a process for the production of perchloroethylene that consumes carbon tetrachloride and minimizes formation of heavy ends.
Direct thermal chlorination of methane, ethane, propane, ethylene, propylene, or their partially chlorinated derivatives exemplifies a conventional process for perchloroethylene production. The chemical reactions for the chlorination of these hydrocarbons and/or their partially chlorinated derivatives are exothermic. They can therefore result in carbon formation or result in an explosion from a runaway reaction if the temperature of the reaction is not controlled. One method of controlling the temperature in the reaction zone is to add a coolant or diluent to the feed mixture. A diluent is defined as any material that is injected into the reactor in order to moderate or control the reactor temperature. The use of vaporized carbon tetrachloride as a diluent to control reactor temperature is disclosed, for example in U.S. Pat. Nos. 2,577,388 and 2,442,323. These patents also disclose recycling of reaction products and use of other variables to control the ratio of carbon tetrachloride to perchloroethylene in the product stream. Use of a liquid diluent made up of chlorinated aliphatic compound such as carbon tetrachloride, perchloroethylene, hexachloroethane, hexachlorobutadiene, and mixtures thereof, is disclosed in U.S. Pat. No. 2,857,438.
It is also known to produce perchloroethylene by pyrolysis of carbon tetrachloride at high temperatures, as disclosed in U.S. Pat. No. 1,930,350. As disclosed in U.S. Pat. No. 3,364,272, the pyrolyric process for production of perchloroethylene ordinarily requires temperatures of the order of 800.degree. C. The pyrolysis of carbon tetrachloride disclosed in U.S. Pat. No. 2,447,410 requires a temperature of 1300.degree. C. to 1400.degree. C. The production of perchloroethylene at these high temperatures, however, has serious disadvantages. Notably, the use of such high temperatures generally requires high energy input to initiate and maintain the reaction, expensive materials for reactor construction, and elaborate product separation to remove the unwanted heavy ends.
Catalytic systems have also been used to produce perchloroethylene. U.S. Pat. No. 4,002,695, for instance, discloses a process for preparing perchloroethylene by reaction of carbon tetrachloride vapor with hydrogen in the presence of a barium chloride catalyst at a temperature of at least 500.degree. C. However, the catalysts required in this method are expensive and are subject to deactivation due to fouling with carbon. Coincidentally, the production of unwanted heavy ends, such as hexachloroethane, hexachlorobutadiene and hexachlorobenzene, is promoted by the use of such a catalyst.